The present invention relates to a workpiece machining system, and more particularly to a workpiece machining system including machining apparatus for simultaneously machining the opposite sides of a disc for a disc brake while rotating the disc which lies in a horizontal plane, the workpiece machining system having means for conveying the disc in the horizontal plane from a workpiece inlet conveyor line to the machining apparatus and also to a workpiece outlet conveyor line, so that the disc can be conveyed easily in a short period of time and can be machined efficiently.
Line production processes are widely employed to machine a large number of workpieces efficiently. According to the line production processes, a variety of machining apparatus are disposed alongside of a machining line, and workpieces that are conveyed one by one to the machining line are machined successively by the machining apparatus.
One conventional apparatus for machining the opposite surfaces of a relatively thin workpiece such as a disc for a disc brake is disclosed in Japanese Laid-Open Patent Publication No. 59-24940. In the disclosed arrangement, a robot which can be displaced in a predetermined direction is disposed between the machining apparatus and a conveyor line which conveys a disc-shaped workpiece in a horizontal plane with a pallet. The workpiece is gripped by a hand attached to the distal end of an arm of the robot, and then the arm is turned and at the same time the robot is displaced in the predetermined direction to orient the workpiece vertically and mount the workpiece on the machining apparatus, which then machines the workpiece.
As described above, the workpiece which has been fed in the horizontal plane along the conveyor line has to be oriented from the horizontal position to the vertical position, installed on the working apparatus. Then, the machined workpiece has to be reoriented to the vertical position before it is placed on a pallet on the conveyor line. Therefore, the procedure for controlling the robot to transfer the workpiece between the conveyor line and the machining apparatus and to orient and reorient the workpiece is considerably complex, and it is time-consuming to convey workpieces through the conveyor line and the machining apparatus. Thus, the conventional system cannot meet the demand for machining a large number of workpieces efficiently.
If a workpiece is a brake disc, then the opposite surfaces thereof have to be machined. The above prior system consumes a considerable period of time in machining the workpiece.
Various machining apparatus have been proposed for simultaneously machining the opposite surfaces of a workpiece with a view to increasing machining efficiency. One example of such machining apparatus is shown in Japanese Laid Open Patent Publication No. 59-37002. According to the disclosed apparatus, a workpiece is vertically mounted on the distal end of a horizontally extending spindle by means of a chuck mechanism, and a pair of cutting tools which are horizontally movable toward and away from each other is brought into engagement with the opposite surfaces, respectively, of the workpiece which is being rotated by the spindle, thus simultaneously machining the opposite surfaces of the workpiece.
With this conventional system, however, it is necessary to orient a workpiece which has been conveyed in a horizontal plane on a conveyor line into a vertical position, and install the vertical workpiece on the spindle, and after the workpiece has been machined, it is necessary to detach the workpiece from the spindle, reorient the workpiece into a horizontal position, and then deliver the horizontal workpiece onto the conveyor line. Accordingly, a transfer device for transferring the workpiece between the conveyor line and the machining apparatus is complex in structure and operation, expensive to manufacture, and cannot rapidly transfer the workpiece. As a result, the efficiency of the entire machining process is not increased.
The workpiece is gripped only at its central portion by a chuck mechanism while the workpiece is being machined. Where an inner wall around a small central hole of a workpiece is gripped, as is the case with a disc for a disc brake which has a central hole that is considerably small in diameter as compared with the diameter of the entire disc, the disc tends to be vibrated due to resistance to the cutting of the disc. The outer peripheral edge of the disc may be held by chuck fingers or the like, but those areas of the disc which are gripped by the chuck fingers are not machined.
In the above prior apparatus, two cutting tools are mounted respectively on tool bases which are threaded over a single screw shaft and displaceable in opposite directions upon rotation of the screw shaft about its own axis. Practically, it is highly difficult to keep the two cutting tools spaced from each other by a highly accurate interval. While the workpiece is being machined, the tool bases are moved by resistance applied to the cutting tools, with the result that the workpiece cannot be machined with high accuracy.
Since the spindle extends horizontally and the tool bases are movable toward and away from each other along the axis of the spindle, the machining apparatus is large in overall size. If a number of such machining apparatus are combined for machining a large number of workpieces efficiently, the entire system takes up a considerably large space.
For transferring a relatively thin workpiece such as a disc for a disc brake between a conveyor line and a machining apparatus, there may be employed a first feed device for pressing and holding an inner wall of the workpiece around a central hole defined therein and a second feed device for pressing and gripping an outer peripheral wall of the workpiece with retractable fingers, as disclosed in Japanese Laid-Open Patent Publication No. 60-15094, for example.
However, because the central hole of the workpiece is of a relatively small diameter as compared with the diameter of the entire workpiece, the workpiece is liable to drop from the first feed device. Furthermore, the workpiece may not be centered accurately in the second feed device. As a consequence, the workpiece may not be transferred reliably to the machining apparatus, and may fail to be properly installed on the machining apparatus.